The present invention relates generally to fault indicators for alternating current electrical distribution systems, and more particularly to fault indicators having improved high impedance trip inhibiting circuitry wherein the indicators are disabled for a predetermined period of time following power-up of a monitored conductor.
Fault indicators of various types have been constructed for detecting faults in electrical power distribution systems. Such indicators include clamp-on type indicators, which clamp directly over cables in the system, and test point-type indicators, which are mounted on test points provided on connectors or components of the system. Fault indicators of both types may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indication is reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Ill., and in U.S. Pat. Nos. 4,063,171, 4,234,847, 4,251,770, 4,236,550 4,438,03 and 4,458,198 of the present inventor.
Since fault conditions in a distribution system may be transitory in nature, as, for example, when a tree branch momentarily contacts a high voltage conductor, distribution circuits are frequently provided with automatic reclosers. In such circuits, the occurrence of a transitory fault results in the circuit interrupter supplying the affected circuit first opening, and then after a short interval trying to reclose to establish power in the circuit. If following reclosure the current is within normal limits, as would be the case if the fault condition was transitory and thus no longer present, the circuit interrupter remains closed and power is distributed through the associated branch of the system. If the fault condition is not transitory, and hence is still present at the time of the automatic recloser cycling, the circuit breaker again trips and after a predetermined number of such attempts power is removed from the circuit.
Upon each cycling of the interrupter, the momentary inrush or initial transient current in the system may exceed normal system current limits. In practice, the inrush current may reach 10 to 20 times the normal steady state current of the conductor and may last for one-half cycle. This may result in fault indicators installed in circuit branches switched by the circuit interrupter but not affected by the original fault condition being falsely trapped.
A fault indicator providing protection against such inrush current is described in the copending application of the present inventor, entitled "Fault Indicator Having Trip Inhibit Circuit", U.S. Pat. No. 4,686,518, issued Aug. 11, 1987. In this indicator, which incorporates loss of voltage circuitry similar to that described in U.S. Pat. No. 4,550,288 of the present inventor, the trip function is disabled for a period immediately following re-energization of the monitored circuit to prevent triggering by the inrush current. The present invention is directed to an improvement in such trip-inhibited fault indicators wherein a high impedance loss of voltage detector circuit, similar to that described in the copending application of the present inventor entitled "Voltage Loss Detector", U.S. Pat. No. 4,714,916, issued Dec. 22, 1987 incorporated in the indicator. As a result, the improved indicator has a substantially higher input impedance and a reduced response and recovery time, making the indicator more suitable for use in demanding situations.
Accordingly, it is a general object of the present invention to provide a new and improved trip-inhibited fault indicator.
It is a more specific object of the present invention to provide a new and improved fault indicator which incorporates improved high impedance circuitry rendering the fault indicator insensitive to inrush current following restoration of current in a monitored conductor.